Radiology #9: Pulling a FAST one

The Patient

A 5 year old child presents following a high speed motor vehicle collision. They were restrained, and are complaining of abdominal pain. Their vital signs are within normal limits, and there is no peritonism. A colleague of yours has performed a FAST scan - the images of which can be seen below:

The Pictures

On the basis of these images, can we discharge the patient?

click to reveal answer

If you've been left scratching your head trying to find the haemoperitoneum, you're in good company. Obviously we have omitted the longitudinal suprapubic view, and from a technical standpoint there are more structures we should have demonstrated - like the liver tip - but on the basis of these saved images, the scan is negative for any intra-abdominal free fluid.

However, for a number of reasons, this doesn't provide much reassurance.

Firstly, a negative FAST doesn't mean no intra-abdominal injury - in anyone, children or adults. Patients can still have injury without haemoperitoneum. Children in particular are more likely to have contained solid organ injury - meaning that these injuries won't contribute to the development of any visible haemoperitoneum [1]. Another aspect to consider is simply the mathematics of it - the expected weight for a 5 year old is approximately 18kg. A child this size will have a circulating blood volume of about 1.3L (assuming a circulating blood volume of 70-80mL/kg). Studies looking at sensitivity in adults vary wildly (sensitivity will depend on the patient's habitus, volume of haemoperitoneum, the patient's haemodynamic stability, and the skills of the clinician) - but the consensus seems to be that in adults, we will have roughly 90% sensitivity once there's 500mL of haemoperitoneum [2]. This volume represents 10% of an adult's circulating blood volume, but would represent about 40% of this child's - by which stage we should be seeing some haemodynamic instability. This means the child's abdomen can "hide" a larger proportion of their circulating blood volume before our FAST becomes positive. This is supported by the literature - one large prospective trial of injured children (Calder et al., n=2188) found FAST to be just 27.8% sensitive [3], and another (Fox et al., n=357) found it to be 20% sensitive [4].

So that means that a negative FAST in a stable child isn't that reassuring. It hasn't significantly changed our post-test probability of intra-abdominal injury.

But what if the scan was positive? Firstly, children often have some quantity of physiological free fluid [5], but more importantly, would finding haemoperitoneum significantly change our management? Solid organ injury in children is typically managed non-operatively, even in the context of haemodynamic instability. Calder et al. found that, of the 27 children in their study who had a true positive FAST - only 12 received intervention - all of whom had an abnormal abdominal exam, and all of whom proceeded to CT prior to the operating theatre [3].

In summary - it's not particularly reassuring when it's negative, and it's not particularly contributory when it's positive.

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Thanks to Dr Ryoko Kinukawa for submitting this case.

References

[1] Coley BD, Mutabagani KH, Martin LC, Zumberge N, Cooney DR, et al. Focused abdominal sonography for trauma (FAST) in children with blunt abdominal trauma. J Trauma. 2000;48(5):902-6.

[2] Focused Assessment with Sonography for Trauma (FAST) scan. Radiopaedia [internet]. 2026. Accessed 21 Mar 2026. Available from: https://radiopaedia.org/articles/focused-assessment-with-sonography-for-trauma-fast-scan

[3] Calder BW, Vogel AM, Zhang J, Mauldin PD, Huang EY, et al. Focused assessment with sonography for trauma in children after blunt abdominal trauma: a multi-institutional analysis. J Trauma Acute Care Surg. 2017;83(2):218-24.

[4] Fox JC, Boysen M, Gharahbaghian L, Cusick S, Ahmed SS, et al. Test characteristics of focused assessment of sonography for trauma for clinically significant abdominal free fluid in pediatric blunt abdominal trauma. Acad Emerg Med. 2011;18(5):477-82.

[5] Berona K, Kang T, Rose E. Pelvic free fluid in asymptomatic pediatric blunt abdominal trauma patients: a case series and review of the literature. J Emerg Med. 2016;50(5):753-8.